from reportlab.lib.pagesizes import A4
from reportlab.lib import colors
from reportlab.lib.units import cm
from reportlab.lib.styles import getSampleStyleSheet, ParagraphStyle
from reportlab.lib.enums import TA_LEFT, TA_CENTER, TA_JUSTIFY
from reportlab.platypus import (
    SimpleDocTemplate, Paragraph, Spacer, Table, TableStyle,
    HRFlowable, PageBreak
)
from reportlab.platypus.flowables import KeepTogether

OUTPUT = "/home/daytona/workspace/vbg-pdf/Venous_Blood_Gases.pdf"

doc = SimpleDocTemplate(
    OUTPUT,
    pagesize=A4,
    leftMargin=2*cm, rightMargin=2*cm,
    topMargin=2.2*cm, bottomMargin=2.2*cm,
    title="Venous Blood Gases",
    author="Orris Medical Reference"
)

W = A4[0] - 4*cm  # usable width

# ── Colour palette ──────────────────────────────────────────────────────────
NAVY   = colors.HexColor("#1a3a5c")
TEAL   = colors.HexColor("#0d6e8a")
LIGHT  = colors.HexColor("#e8f4f8")
ACCENT = colors.HexColor("#f0a500")
WHITE  = colors.white
LGREY  = colors.HexColor("#f5f5f5")
DGREY  = colors.HexColor("#333333")
MGREY  = colors.HexColor("#666666")
RED    = colors.HexColor("#c0392b")
GREEN  = colors.HexColor("#1a7a4a")

# ── Styles ───────────────────────────────────────────────────────────────────
styles = getSampleStyleSheet()

title_style = ParagraphStyle("Title", fontName="Helvetica-Bold",
    fontSize=22, textColor=WHITE, alignment=TA_CENTER, leading=28)
subtitle_style = ParagraphStyle("Subtitle", fontName="Helvetica",
    fontSize=11, textColor=colors.HexColor("#cce8f0"),
    alignment=TA_CENTER, leading=16)

h1 = ParagraphStyle("H1", fontName="Helvetica-Bold",
    fontSize=14, textColor=WHITE, leading=18,
    backColor=NAVY, leftPadding=8, rightPadding=8,
    spaceBefore=14, spaceAfter=4)
h2 = ParagraphStyle("H2", fontName="Helvetica-Bold",
    fontSize=12, textColor=TEAL, leading=16,
    spaceBefore=10, spaceAfter=3)
body = ParagraphStyle("Body", fontName="Helvetica",
    fontSize=9.5, textColor=DGREY, leading=14,
    spaceBefore=2, spaceAfter=2, alignment=TA_JUSTIFY)
bullet = ParagraphStyle("Bullet", fontName="Helvetica",
    fontSize=9.5, textColor=DGREY, leading=14,
    leftIndent=14, bulletIndent=4, spaceBefore=1, spaceAfter=1)
small_italic = ParagraphStyle("SmallItalic", fontName="Helvetica-Oblique",
    fontSize=8.5, textColor=MGREY, leading=12,
    spaceBefore=2, spaceAfter=4)
note_style = ParagraphStyle("Note", fontName="Helvetica",
    fontSize=8.8, textColor=DGREY, leading=13,
    backColor=LIGHT, leftPadding=8, rightPadding=8, borderPadding=6,
    spaceBefore=4, spaceAfter=4)
warn_style = ParagraphStyle("Warn", fontName="Helvetica-Bold",
    fontSize=9, textColor=RED, leading=13,
    leftIndent=10, spaceBefore=2, spaceAfter=2)
source_style = ParagraphStyle("Source", fontName="Helvetica-Oblique",
    fontSize=8, textColor=MGREY, leading=11,
    spaceBefore=0, spaceAfter=0)

def h1_para(text):
    return Paragraph(f" {text}", h1)

def h2_para(text):
    return Paragraph(text, h2)

def para(text):
    return Paragraph(text, body)

def bul(text):
    return Paragraph(f"\u2022  {text}", bullet)

def src(text):
    return Paragraph(f"<i>- {text}</i>", source_style)


# ── Helper: coloured table ───────────────────────────────────────────────────
def make_table(headers, rows, col_widths=None, header_bg=NAVY, alt=True):
    data = [[Paragraph(f"<b>{h}</b>", ParagraphStyle("TH",
                fontName="Helvetica-Bold", fontSize=9, textColor=WHITE,
                leading=12, alignment=TA_CENTER))
             for h in headers]]
    for i, row in enumerate(rows):
        data.append([Paragraph(str(cell), ParagraphStyle("TD",
                        fontName="Helvetica", fontSize=8.8,
                        textColor=DGREY, leading=12, alignment=TA_CENTER))
                     for cell in row])

    style = [
        ("BACKGROUND", (0,0), (-1,0), header_bg),
        ("ROWBACKGROUNDS", (0,1), (-1,-1),
         [LGREY, WHITE] if alt else [WHITE]),
        ("GRID", (0,0), (-1,-1), 0.4, colors.HexColor("#cccccc")),
        ("VALIGN", (0,0), (-1,-1), "MIDDLE"),
        ("TOPPADDING", (0,0), (-1,-1), 4),
        ("BOTTOMPADDING", (0,0), (-1,-1), 4),
        ("LEFTPADDING", (0,0), (-1,-1), 6),
        ("RIGHTPADDING", (0,0), (-1,-1), 6),
        ("LINEBELOW", (0,0), (-1,0), 1, ACCENT),
    ]
    t = Table(data, colWidths=col_widths, repeatRows=1)
    t.setStyle(TableStyle(style))
    return t


# ── Build story ──────────────────────────────────────────────────────────────
story = []

# ── Cover banner ─────────────────────────────────────────────────────────────
banner_data = [[
    Paragraph("Venous Blood Gases", title_style),
    ""
],[
    Paragraph("A Comprehensive Clinical Reference", subtitle_style),
    ""
],[
    Paragraph("Based on: Tintinalli's EM · Murray &amp; Nadel's · Goldman-Cecil · Fishman's · Miller's Anesthesia · Rosen's EM", subtitle_style),
    ""
]]
banner = Table(banner_data, colWidths=[W, 0])
banner.setStyle(TableStyle([
    ("BACKGROUND", (0,0), (-1,-1), NAVY),
    ("TOPPADDING", (0,0), (-1,-1), 10),
    ("BOTTOMPADDING", (0,0), (-1,-1), 10),
    ("LEFTPADDING", (0,0), (-1,-1), 12),
    ("RIGHTPADDING", (0,0), (-1,-1), 12),
]))
story.append(banner)
story.append(Spacer(1, 0.5*cm))

# ── 1. Definition ─────────────────────────────────────────────────────────────
story.append(h1_para("1.  What Is a Venous Blood Gas (VBG)?"))
story.append(Spacer(1, 0.15*cm))
story.append(para(
    "A <b>venous blood gas (VBG)</b> is an analysis of blood taken from a venous source "
    "to measure pH, PCO\u2082, PO\u2082, bicarbonate (HCO\u2083\u207b), and oxygen saturation. "
    "It is less invasive and less painful than an arterial blood gas (ABG), making it widely "
    "used in emergency medicine and critical care. Partially or centrally obtained venous "
    "samples are both in routine clinical use."
))
story.append(src("Tintinalli's Emergency Medicine, p. 304 | Murray & Nadel's Textbook of Respiratory Medicine, p. 2280"))
story.append(Spacer(1, 0.2*cm))

# ── 2. Sampling Sites ────────────────────────────────────────────────────────
story.append(h1_para("2.  Sampling Sites"))
story.append(Spacer(1, 0.15*cm))
story.append(make_table(
    ["Site", "Source", "Clinical Use"],
    [
        ["Peripheral vein", "Arm / hand vein", "Most common; easy access; widely used in ED"],
        ["Central vein (SVC / right atrium)", "Central venous catheter",
         "Better systemic assessment; ScvO\u2082 monitoring"],
        ["Pulmonary artery (mixed venous)", "Pulmonary artery catheter (PAC)",
         "Gold standard for true mixed venous; reflects global O\u2082 balance"],
    ],
    col_widths=[W*0.28, W*0.28, W*0.44]
))
story.append(Spacer(1, 0.2*cm))
story.append(bul(
    "The <b>pulmonary artery</b> is the ideal site because blood from all body sites is equally "
    "represented, but placement is rarely practical."
))
story.append(bul(
    "Blood from the <b>SVC</b> disproportionately represents cerebral and upper body flow."
))
story.append(bul(
    "<b>Peripheral venous</b> samples are widely used and correlate closely enough to be "
    "clinically useful; significantly abnormal values should be confirmed with an ABG."
))
story.append(src("Tintinalli's Emergency Medicine, p. 304"))
story.append(Spacer(1, 0.2*cm))

# ── 3. Normal Values ─────────────────────────────────────────────────────────
story.append(h1_para("3.  Normal VBG Values vs. ABG Values"))
story.append(Spacer(1, 0.15*cm))
story.append(make_table(
    ["Parameter", "Arterial (ABG)", "Venous (VBG)", "Typical Difference"],
    [
        ["pH", "7.35 - 7.45", "~7.32 - 7.42", "~0.03 - 0.05 lower"],
        ["PCO\u2082 (mmHg)", "35 - 45", "~40 - 50", "~3 - 8 mmHg higher"],
        ["PO\u2082 (mmHg)", "80 - 100", "~35 - 45", "Much lower (not for oxygenation)"],
        ["HCO\u2083\u207b (mEq/L)", "22 - 26", "~24 - 29", "~2 - 3 mEq/L higher"],
        ["O\u2082 Saturation", "95 - 100%", "~60 - 80% (SvO\u2082)", "Reflects tissue O\u2082 extraction"],
    ],
    col_widths=[W*0.22, W*0.22, W*0.22, W*0.34]
))
story.append(Spacer(1, 0.2*cm))
story.append(bul(
    "Venous <b>HCO\u2083\u207b</b> runs ~2-3 mmol/L higher because venous blood includes CO\u2082 "
    "from cellular metabolism not yet excreted by the lungs, plus carbonic acid, dissolved CO\u2082, "
    "carbonate, and carbamates."
))
story.append(src("Murray & Nadel's, p. 2280-2282 | Goldman-Cecil Medicine, p. 754"))
story.append(Spacer(1, 0.2*cm))

# ── 4. What VBG Can and Cannot Tell You ──────────────────────────────────────
story.append(h1_para("4.  What VBG Can and Cannot Tell You"))
story.append(Spacer(1, 0.15*cm))

story.append(h2_para("CAN Use VBG For:"))
story.append(bul(
    "<b>pH assessment:</b> Correlates closely with arterial pH (±0.03-0.05 units). In most "
    "clinical scenarios this difference is not clinically significant. Central VBGs are more "
    "accurate than peripheral."
))
story.append(bul(
    "<b>Hypercapnia screening:</b> A <b>normal PvCO\u2082 effectively excludes hypercapnic "
    "respiratory failure.</b> If venous PCO\u2082 is normal, arterial PCO\u2082 is almost "
    "certainly normal."
))
story.append(bul(
    "<b>Acid-base disorders:</b> Excellent agreement with ABG for detecting acid-base disorders, "
    "including in ICU patients in shock."
))
story.append(bul(
    "<b>Bicarbonate estimation:</b> Venous total CO\u2082/HCO\u2083\u207b is a useful surrogate "
    "for arterial HCO\u2083\u207b."
))
story.append(bul(
    "<b>Lactate (screening):</b> Normal and markedly abnormal venous lactate correlates with "
    "arterial lactate. Mildly elevated venous lactate may not reliably correlate - confirm with "
    "arterial if clinically important."
))
story.append(bul(
    "<b>DKA monitoring:</b> VBG correlates well with ABG for pH and HCO\u2083\u207b, avoiding "
    "repeated arterial punctures."
))

story.append(Spacer(1, 0.15*cm))
story.append(h2_para("CANNOT Use VBG For:"))
story.append(Paragraph(
    "\u26a0\ufe0f  <b>Oxygenation assessment:</b> Venous PO\u2082 does NOT correlate with arterial "
    "oxygen content and <b>cannot</b> be used to assess hypoxemia. ABG or SpO\u2082 is required.",
    ParagraphStyle("WarnBox", fontName="Helvetica-Bold", fontSize=9.5,
        textColor=RED, leading=14, leftIndent=14, spaceBefore=3, spaceAfter=3)
))
story.append(Paragraph(
    "\u26a0\ufe0f  <b>Reliable hypercarbia quantification when severe:</b> In low cardiac output, "
    "high CO\u2082 production, or inhibition of red cell carbonic anhydrase, the arteriovenous "
    "PCO\u2082 difference can increase up to <b>10-fold</b>, rendering VBG unreliable.",
    ParagraphStyle("WarnBox2", fontName="Helvetica-Bold", fontSize=9.5,
        textColor=RED, leading=14, leftIndent=14, spaceBefore=3, spaceAfter=3)
))
story.append(Paragraph(
    "\u26a0\ufe0f  <b>Hypotensive patients with severe hypercapnia:</b> VBGs are considered "
    "unacceptably inaccurate. ABG is preferred.",
    ParagraphStyle("WarnBox3", fontName="Helvetica-Bold", fontSize=9.5,
        textColor=RED, leading=14, leftIndent=14, spaceBefore=3, spaceAfter=3)
))
story.append(src("Goldman-Cecil, p. 754 | Tintinalli's, p. 306 | Murray & Nadel's, p. 2280"))
story.append(Spacer(1, 0.2*cm))

# ── 5. Mixed Venous O2 Saturation ────────────────────────────────────────────
story.append(h1_para("5.  Mixed Venous Oxygen Saturation (SvO\u2082)"))
story.append(Spacer(1, 0.15*cm))
story.append(para(
    "The <b>mixed venous oxygen saturation (SvO\u2082)</b>, measured from the pulmonary artery "
    "via a PAC, is a global indicator of the balance between oxygen delivery (DO\u2082) and "
    "oxygen consumption (VO\u2082)."
))
story.append(Spacer(1, 0.1*cm))
story.append(make_table(
    ["SvO\u2082 Value", "Interpretation"],
    [
        ["Normal: 65 - 75%", "Adequate global O\u2082 delivery/consumption balance"],
        ["Low: < 65%", "Global O\u2082 delivery deficient - low CO, severe anaemia, high metabolic state"],
        ["High: > 75 - 80%", "Distributive shock (e.g. sepsis), impaired tissue O\u2082 extraction, or high-flow state"],
    ],
    col_widths=[W*0.30, W*0.70]
))
story.append(Spacer(1, 0.15*cm))
story.append(para(
    "<b>Fick's principle</b> allows cardiac output estimation using SvO\u2082:"
))
# Formula box
formula_table = Table(
    [[Paragraph("CO = VO\u2082 / (CaO\u2082 \u2212 CvO\u2082)",
        ParagraphStyle("Formula", fontName="Helvetica-Bold", fontSize=11,
            textColor=NAVY, leading=16, alignment=TA_CENTER))]],
    colWidths=[W]
)
formula_table.setStyle(TableStyle([
    ("BACKGROUND", (0,0), (-1,-1), LIGHT),
    ("BOX", (0,0), (-1,-1), 1, TEAL),
    ("TOPPADDING", (0,0), (-1,-1), 8),
    ("BOTTOMPADDING", (0,0), (-1,-1), 8),
]))
story.append(formula_table)
story.append(Spacer(1, 0.1*cm))
story.append(bul(
    "<b>Important limitation:</b> SvO\u2082 is a <i>global</i> measure. Organ-specific ischemia "
    "may be present even with a normal or elevated mixed venous saturation."
))
story.append(src("Miller's Anesthesia 10e, p. 833-835"))
story.append(Spacer(1, 0.2*cm))

# ScvO2 vs SvO2
story.append(h2_para("Central Venous O\u2082 Saturation (ScvO\u2082) vs. Mixed Venous (SvO\u2082)"))
story.append(make_table(
    ["", "ScvO\u2082 (SVC / right atrium)", "SvO\u2082 (pulmonary artery)"],
    [
        ["Access", "Central venous catheter", "Pulmonary artery catheter (PAC)"],
        ["Represents", "Upper body + cerebral flow", "Entire body"],
        ["Normal value", "~70 - 80%", "~65 - 75%"],
        ["Reliability", "Variable; NOT a reliable SvO\u2082 surrogate", "Gold standard"],
    ],
    col_widths=[W*0.22, W*0.39, W*0.39]
))
story.append(Spacer(1, 0.1*cm))
story.append(bul(
    "Monitoring of ScvO\u2082 was previously part of the <b>Surviving Sepsis Campaign</b>, but "
    "failure to demonstrate clinical benefit led to its <b>removal from the 2016 guidelines</b>."
))
story.append(src("Tintinalli's, p. 308 | Miller's Anesthesia, p. 835"))
story.append(Spacer(1, 0.2*cm))

# ── 6. A-V Differences ───────────────────────────────────────────────────────
story.append(h1_para("6.  Arteriovenous Differences - Why They Exist"))
story.append(Spacer(1, 0.15*cm))
story.append(para(
    "Venous blood is lower in O\u2082 and higher in CO\u2082 because tissues extract oxygen and "
    "produce CO\u2082. Normal arteriovenous differences:"
))
story.append(Spacer(1, 0.1*cm))
story.append(make_table(
    ["Parameter", "Arterial", "Venous", "Normal A-V Difference"],
    [
        ["pH", "7.35 - 7.45", "~0.03 - 0.05 lower", "Vein is more acidic"],
        ["PCO\u2082 (mmHg)", "35 - 45", "~40 - 50", "3 - 8 mmHg higher venously"],
        ["PO\u2082 (mmHg)", "80 - 100", "~35 - 45", "~50 - 60 mmHg lower venously"],
        ["HCO\u2083\u207b (mEq/L)", "22 - 26", "~24 - 29", "~2 - 3 mEq/L higher venously"],
    ],
    col_widths=[W*0.22, W*0.22, W*0.22, W*0.34]
))
story.append(Spacer(1, 0.1*cm))
story.append(bul(
    "These differences <b>widen significantly in low-flow states</b> (shock, heart failure) "
    "because tissues extract proportionally more O\u2082 and dump more CO\u2082."
))
story.append(Spacer(1, 0.2*cm))

# ── 7. Pre-Analytical Errors ─────────────────────────────────────────────────
story.append(h1_para("7.  Pre-Analytical Errors Affecting VBG Accuracy"))
story.append(Spacer(1, 0.15*cm))
story.append(make_table(
    ["Error", "Effect on Values"],
    [
        ["Air exposure", "Decreases PCO\u2082, raises pH, gradually decreases CO\u2082 content"],
        ["Saline / fluid dilution (from flush line)", "PCO\u2082 and HCO\u2083\u207b both fall equally"],
        ["Hypothermia (no temperature correction)", "Spuriously higher PCO\u2082, lower pH, higher PO\u2082"],
        ["Hyperthermia (no temperature correction)", "Opposite of hypothermia effects"],
        ["Delayed analysis (prolonged sample time)", "Continued cellular metabolism alters values"],
    ],
    col_widths=[W*0.45, W*0.55]
))
story.append(src("Murray & Nadel's, p. 2274-2278"))
story.append(Spacer(1, 0.2*cm))

# ── 8. Clinical Applications ─────────────────────────────────────────────────
story.append(h1_para("8.  Clinical Applications"))
story.append(Spacer(1, 0.15*cm))

story.append(h2_para("Emergency Medicine"))
for pt in [
    "<b>Respiratory failure screening:</b> A normal venous PCO\u2082 excludes hypercapnic failure. Severe hypercapnia or hypoxia - confirm with ABG.",
    "<b>DKA monitoring:</b> VBG correlates well with ABG for pH/HCO\u2083\u207b, avoiding repeated arterial punctures.",
    "<b>Toxicology:</b> VBG used alongside electrolytes in phenol exposure, toxic ingestions.",
    "<b>Paediatric assessment:</b> VBG used with glucose, ammonia, and metabolic panel in metabolic emergencies.",
]:
    story.append(bul(pt))

story.append(Spacer(1, 0.1*cm))
story.append(h2_para("Critical Care / ICU"))
for pt in [
    "<b>Acid-base monitoring:</b> Central VBGs have excellent agreement with ABGs for acid-base assessment, including in patients in shock.",
    "<b>Sepsis:</b> ScvO\u2082 monitoring (no longer a Surviving Sepsis Campaign recommendation as of 2016).",
    "<b>Haemorrhagic shock:</b> VBG alongside TEG and type-and-screen is prioritised in severe haemorrhagic shock.",
    "<b>COPD exacerbation:</b> VBG recommended as initial blood gas; follow with ABG if severe hypercapnia or hypoxia.",
]:
    story.append(bul(pt))

story.append(Spacer(1, 0.1*cm))
story.append(h2_para("Surgery / Anaesthesia"))
story.append(bul(
    "Continuous SvO\u2082 monitoring via PAC in high-risk surgical patients provides real-time "
    "assessment of global perfusion adequacy."
))
story.append(src("Tintinalli's, p. 304-310 | Rosen's EM, p. 2550 | Fishman's Pulmonary Diseases, p. 2060-2061"))
story.append(Spacer(1, 0.2*cm))

# ── 9. VBG vs ABG Decision Table ─────────────────────────────────────────────
story.append(h1_para("9.  VBG vs. ABG - When to Choose Which"))
story.append(Spacer(1, 0.15*cm))
story.append(make_table(
    ["Clinical Question", "Recommended Test"],
    [
        ["Is oxygenation adequate?", "ABG or SpO\u2082 - VBG cannot assess this"],
        ["Is the patient hypercapnic?", "VBG first; normal excludes hypercapnia. Confirm ABG if elevated"],
        ["What is the acid-base status?", "VBG is acceptable (pH \u00b10.03-0.05)"],
        ["Is bicarbonate normal?", "VBG (HCO\u2083\u207b ~2-3 mEq/L higher than arterial)"],
        ["Is lactate elevated?", "VBG for screening; mildly elevated should be confirmed with ABG"],
        ["Is global O\u2082 delivery adequate?", "Mixed SvO\u2082 from PAC (or ScvO\u2082 as a trend)"],
        ["Hypotensive / severely hypercapnic patient", "ABG preferred; VBG unreliable in this setting"],
    ],
    col_widths=[W*0.50, W*0.50]
))
story.append(Spacer(1, 0.2*cm))

# ── 10. Key Formulas ─────────────────────────────────────────────────────────
story.append(h1_para("10. Key Formulas"))
story.append(Spacer(1, 0.15*cm))

formulas = [
    ("Winter's Equation", "Expected PaCO\u2082 in metabolic acidosis",
     "PaCO\u2082 = (1.5 \u00d7 [HCO\u2083\u207b] + 8) \u00b1 2",
     "PaCO\u2082 above expected = inadequate ventilatory response"),
    ("Fick Equation", "Cardiac output from mixed venous saturation",
     "CO = VO\u2082 / (CaO\u2082 \u2212 CvO\u2082)",
     "Requires SvO\u2082 from pulmonary artery"),
    ("Dead Space Fraction", "Requires ABG; gauges severity of lung disease",
     "V\u1d05/V\u1d1b = (PaCO\u2082 \u2212 PeCO\u2082) / PaCO\u2082",
     "PeCO\u2082 measured by metabolic monitor or capnography"),
]

for name, desc, formula, note in formulas:
    row_data = [[
        Paragraph(f"<b>{name}</b>",
            ParagraphStyle("FN", fontName="Helvetica-Bold", fontSize=9.5,
                textColor=NAVY, leading=13)),
        Paragraph(desc,
            ParagraphStyle("FD", fontName="Helvetica", fontSize=9,
                textColor=MGREY, leading=13)),
        Paragraph(formula,
            ParagraphStyle("FF", fontName="Helvetica-Bold", fontSize=10,
                textColor=TEAL, leading=14, alignment=TA_CENTER)),
        Paragraph(f"<i>{note}</i>",
            ParagraphStyle("FNote", fontName="Helvetica-Oblique", fontSize=8.5,
                textColor=MGREY, leading=12)),
    ]]
    t = Table(row_data, colWidths=[W*0.22, W*0.26, W*0.28, W*0.24])
    t.setStyle(TableStyle([
        ("BACKGROUND", (0,0), (-1,-1), LGREY),
        ("BOX", (0,0), (-1,-1), 0.5, colors.HexColor("#cccccc")),
        ("LINEAFTER", (0,0), (2,0), 0.5, colors.HexColor("#cccccc")),
        ("VALIGN", (0,0), (-1,-1), "MIDDLE"),
        ("TOPPADDING", (0,0), (-1,-1), 6),
        ("BOTTOMPADDING", (0,0), (-1,-1), 6),
        ("LEFTPADDING", (0,0), (-1,-1), 8),
        ("RIGHTPADDING", (0,0), (-1,-1), 8),
    ]))
    story.append(t)
    story.append(Spacer(1, 0.1*cm))

story.append(Spacer(1, 0.2*cm))

# ── 11. Summary Table ────────────────────────────────────────────────────────
story.append(h1_para("11. Summary Comparison Table"))
story.append(Spacer(1, 0.15*cm))
story.append(make_table(
    ["Feature", "VBG", "ABG"],
    [
        ["Invasiveness", "Low (venipuncture)", "Higher (arterial puncture)"],
        ["Pain", "Minimal", "Moderate"],
        ["pH accuracy", "Good (\u00b10.03-0.05)", "Reference standard"],
        ["PCO\u2082", "~3-8 mmHg higher than arterial", "Reference"],
        ["PO\u2082 / Oxygenation", "\u274c Not useful", "\u2705 Required for O\u2082 assessment"],
        ["HCO\u2083\u207b", "~2-3 mEq/L higher", "Reference"],
        ["Lactate", "Acceptable (screen); confirm if mildly elevated", "More accurate"],
        ["Use in shock / hypotension", "\u26a0 Unreliable", "\u2705 Preferred"],
        ["Mixed venous O\u2082 (SvO\u2082)", "Via PAC only", "N/A"],
    ],
    col_widths=[W*0.32, W*0.35, W*0.33]
))
story.append(Spacer(1, 0.3*cm))

# ── References ────────────────────────────────────────────────────────────────
story.append(HRFlowable(width=W, thickness=1, color=TEAL))
story.append(Spacer(1, 0.1*cm))
story.append(Paragraph("<b>References</b>", ParagraphStyle("RefHead",
    fontName="Helvetica-Bold", fontSize=10, textColor=NAVY, leading=14)))
story.append(Spacer(1, 0.05*cm))
refs = [
    "Tintinalli's Emergency Medicine: A Comprehensive Study Guide, pp. 304-310",
    "Murray & Nadel's Textbook of Respiratory Medicine, pp. 2270-2282",
    "Goldman-Cecil Medicine International Edition, p. 754",
    "Fishman's Pulmonary Diseases and Disorders, p. 2060",
    "Miller's Anesthesia 10th Edition, pp. 833-835",
    "Rosen's Emergency Medicine: Concepts and Clinical Practice, pp. 2550-2551",
    "Harrison's Principles of Internal Medicine 22E (2025)",
]
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<!-- BANNER -->
<div class="banner">
  <h1>Venous Blood Gases (VBG)</h1>
  <div class="sub">A Comprehensive Clinical Reference</div>
  <div class="sub" style="margin-top:4px; font-size:9pt;">
    Based on: Tintinalli's EM &nbsp;·&nbsp; Murray &amp; Nadel's &nbsp;·&nbsp; Goldman-Cecil &nbsp;·&nbsp; Fishman's &nbsp;·&nbsp; Miller's Anesthesia &nbsp;·&nbsp; Rosen's EM
  </div>
</div>

<!-- 1. DEFINITION -->
<h2>1. What Is a Venous Blood Gas (VBG)?</h2>
<p>
  A <strong>venous blood gas (VBG)</strong> is an analysis of blood taken from a venous source to measure
  pH, PCO<sub>2</sub>, PO<sub>2</sub>, bicarbonate (HCO<sub>3</sub><sup>−</sup>), and oxygen saturation.
  It is less invasive and less painful than an arterial blood gas (ABG), making it widely used in
  emergency medicine and critical care. Both peripherally and centrally obtained venous samples are in
  routine clinical use.
</p>
<p class="src">— Tintinalli's Emergency Medicine, p. 304 &nbsp;|&nbsp; Murray &amp; Nadel's Textbook of Respiratory Medicine, p. 2280</p>

<!-- 2. SAMPLING SITES -->
<h2>2. Sampling Sites</h2>
<table>
  <thead><tr><th>Site</th><th>Source</th><th>Clinical Use</th></tr></thead>
  <tbody>
    <tr>
      <td><strong>Peripheral vein</strong></td>
      <td>Arm / hand vein</td>
      <td>Most common; easy access; widely used in ED</td>
    </tr>
    <tr>
      <td><strong>Central vein (SVC / right atrium)</strong></td>
      <td>Central venous catheter</td>
      <td>Better systemic assessment; ScvO<sub>2</sub> monitoring</td>
    </tr>
    <tr>
      <td><strong>Pulmonary artery (mixed venous)</strong></td>
      <td>Pulmonary artery catheter (PAC)</td>
      <td>Gold standard for true mixed venous; reflects global O<sub>2</sub> balance</td>
    </tr>
  </tbody>
</table>
<ul>
  <li>The <strong>pulmonary artery</strong> is the ideal site because blood from all body sites is equally represented, but it is rarely practical to obtain.</li>
  <li>Blood from the <strong>SVC</strong> disproportionately represents cerebral and upper body blood flow.</li>
  <li><strong>Peripheral venous</strong> samples correlate closely enough to be clinically useful; significantly abnormal values should be confirmed with an ABG.</li>
</ul>
<p class="src">— Tintinalli's Emergency Medicine, p. 304</p>

<!-- 3. NORMAL VALUES -->
<h2>3. Normal VBG Values vs. ABG Values</h2>
<table>
  <thead><tr><th>Parameter</th><th>Arterial (ABG)</th><th>Venous (VBG)</th><th>Typical Difference</th></tr></thead>
  <tbody>
    <tr><td><strong>pH</strong></td><td>7.35 – 7.45</td><td>~7.32 – 7.42</td><td>~0.03 – 0.05 lower</td></tr>
    <tr><td><strong>PCO<sub>2</sub> (mmHg)</strong></td><td>35 – 45</td><td>~40 – 50</td><td>~3 – 8 mmHg higher</td></tr>
    <tr><td><strong>PO<sub>2</sub> (mmHg)</strong></td><td>80 – 100</td><td>~35 – 45</td><td>Much lower (not usable for oxygenation)</td></tr>
    <tr><td><strong>HCO<sub>3</sub><sup>−</sup> (mEq/L)</strong></td><td>22 – 26</td><td>~24 – 29</td><td>~2 – 3 mEq/L higher</td></tr>
    <tr><td><strong>O<sub>2</sub> Saturation</strong></td><td>95 – 100%</td><td>~60 – 80% (SvO<sub>2</sub>)</td><td>Reflects tissue O<sub>2</sub> extraction</td></tr>
  </tbody>
</table>
<ul>
  <li>Venous <strong>HCO<sub>3</sub><sup>−</sup></strong> runs ~2–3 mmol/L higher because venous blood includes CO<sub>2</sub> from cellular metabolism not yet excreted by the lungs, plus carbonic acid, dissolved CO<sub>2</sub>, carbonate, and carbamates.</li>
  <li>Venous pH averages ~0.03 lower than arterial (central VBG); peripheral VBG may differ by up to ±0.05.</li>
</ul>
<p class="src">— Murray &amp; Nadel's, p. 2280–2282 &nbsp;|&nbsp; Goldman-Cecil Medicine, p. 754</p>

<!-- 4. CAN / CANNOT -->
<h2>4. What VBG Can and Cannot Tell You</h2>

<h3>&#10003; CAN Use VBG For:</h3>
<ul>
  <li><strong>pH assessment:</strong> Correlates closely with arterial pH (±0.03–0.05 units). In most clinical scenarios this difference is not clinically significant. Central VBGs are more accurate than peripheral.</li>
  <li><strong>Hypercapnia screening:</strong> A <strong>normal PvCO<sub>2</sub> effectively excludes hypercapnic respiratory failure.</strong> If venous PCO<sub>2</sub> is normal, arterial PCO<sub>2</sub> is almost certainly normal.</li>
  <li><strong>Acid-base disorders:</strong> Excellent agreement with ABG for detecting acid-base disorders, including in ICU patients in shock.</li>
  <li><strong>Bicarbonate estimation:</strong> Venous total CO<sub>2</sub>/HCO<sub>3</sub><sup>−</sup> is a useful surrogate for arterial HCO<sub>3</sub><sup>−</sup>.</li>
  <li><strong>Lactate (screening):</strong> Normal and markedly abnormal venous lactate correlates with arterial lactate. Mildly elevated venous lactate may not reliably correlate — confirm with arterial if clinically important.</li>
  <li><strong>DKA monitoring:</strong> VBG correlates well with ABG for pH and HCO<sub>3</sub><sup>−</sup>, avoiding repeated arterial punctures.</li>
</ul>

<h3>&#10007; CANNOT Use VBG For:</h3>
<div class="warn">&#9888; Oxygenation assessment: Venous PO<sub>2</sub> does NOT correlate with arterial oxygen content and <u>cannot</u> be used to assess hypoxaemia. ABG or SpO<sub>2</sub> is required.</div>
<div class="warn">&#9888; Reliable hypercarbia quantification when severe: In low cardiac output, high CO<sub>2</sub> production, or inhibition of red cell carbonic anhydrase, the arteriovenous PCO<sub>2</sub> difference can increase up to <strong>10-fold</strong>, rendering VBG unreliable.</div>
<div class="warn">&#9888; Hypotensive patients with severe hypercapnia: VBGs are considered unacceptably inaccurate. ABG is preferred.</div>
<p class="src">— Goldman-Cecil, p. 754 &nbsp;|&nbsp; Tintinalli's, p. 306 &nbsp;|&nbsp; Murray &amp; Nadel's, p. 2280</p>

<!-- 5. MIXED VENOUS -->
<h2>5. Mixed Venous Oxygen Saturation (SvO<sub>2</sub>)</h2>
<p>
  The <strong>mixed venous oxygen saturation (SvO<sub>2</sub>)</strong>, measured from the pulmonary artery
  via a PAC, is a global indicator of the balance between oxygen delivery (DO<sub>2</sub>) and oxygen
  consumption (VO<sub>2</sub>).
</p>
<table>
  <thead><tr><th>SvO<sub>2</sub> Value</th><th>Interpretation</th></tr></thead>
  <tbody>
    <tr><td>Normal: 65 – 75%</td><td>Adequate global O<sub>2</sub> delivery/consumption balance</td></tr>
    <tr><td>Low: &lt; 65%</td><td>Global O<sub>2</sub> delivery deficient — low cardiac output, severe anaemia, high metabolic demand</td></tr>
    <tr><td>High: &gt; 75 – 80%</td><td>Distributive shock (e.g. sepsis), impaired tissue O<sub>2</sub> extraction, or high-flow states</td></tr>
  </tbody>
</table>

<p><strong>Fick's Principle</strong> — cardiac output estimation using SvO<sub>2</sub>:</p>
<div class="formula">CO = VO<sub>2</sub> / (CaO<sub>2</sub> − CvO<sub>2</sub>)</div>

<ul>
  <li><strong>Important limitation:</strong> SvO<sub>2</sub> is a <em>global</em> measure. Organ-specific ischaemia may be present even with a normal or elevated mixed venous saturation.</li>
</ul>
<p class="src">— Miller's Anesthesia 10e, p. 833–835</p>

<h3>Central Venous O<sub>2</sub> Saturation (ScvO<sub>2</sub>) vs. Mixed Venous (SvO<sub>2</sub>)</h3>
<table>
  <thead><tr><th></th><th>ScvO<sub>2</sub> (SVC / right atrium)</th><th>SvO<sub>2</sub> (pulmonary artery)</th></tr></thead>
  <tbody>
    <tr><td><strong>Access</strong></td><td>Central venous catheter</td><td>Pulmonary artery catheter (PAC)</td></tr>
    <tr><td><strong>Represents</strong></td><td>Upper body + cerebral flow</td><td>Entire body</td></tr>
    <tr><td><strong>Normal value</strong></td><td>~70 – 80%</td><td>~65 – 75%</td></tr>
    <tr><td><strong>Reliability</strong></td><td>Variable; NOT a reliable SvO<sub>2</sub> surrogate</td><td>Gold standard</td></tr>
  </tbody>
</table>
<ul>
  <li>Monitoring of ScvO<sub>2</sub> was previously part of the <strong>Surviving Sepsis Campaign</strong>, but failure to demonstrate clinical benefit led to its <strong>removal from the 2016 guidelines</strong>.</li>
</ul>
<p class="src">— Tintinalli's, p. 308 &nbsp;|&nbsp; Miller's Anesthesia, p. 835</p>

<!-- 6. A-V DIFFERENCES -->
<h2>6. Arteriovenous Differences — Why They Exist</h2>
<p>
  Venous blood is lower in O<sub>2</sub> and higher in CO<sub>2</sub> because tissues extract oxygen
  and produce CO<sub>2</sub>. These differences widen significantly in low-flow states (shock, heart failure).
</p>
<table>
  <thead><tr><th>Parameter</th><th>Arterial</th><th>Venous</th><th>Normal A-V Difference</th></tr></thead>
  <tbody>
    <tr><td>pH</td><td>7.35 – 7.45</td><td>~0.03–0.05 lower</td><td>Vein is more acidic</td></tr>
    <tr><td>PCO<sub>2</sub> (mmHg)</td><td>35 – 45</td><td>~40 – 50</td><td>3 – 8 mmHg higher venously</td></tr>
    <tr><td>PO<sub>2</sub> (mmHg)</td><td>80 – 100</td><td>~35 – 45</td><td>~50 – 60 mmHg lower venously</td></tr>
    <tr><td>HCO<sub>3</sub><sup>−</sup> (mEq/L)</td><td>22 – 26</td><td>~24 – 29</td><td>~2 – 3 mEq/L higher venously</td></tr>
  </tbody>
</table>

<!-- 7. PRE-ANALYTICAL ERRORS -->
<h2>7. Pre-Analytical Errors Affecting VBG Accuracy</h2>
<table>
  <thead><tr><th>Error</th><th>Effect on Values</th></tr></thead>
  <tbody>
    <tr><td><strong>Air exposure</strong></td><td>Decreases PCO<sub>2</sub>, raises pH, gradually decreases CO<sub>2</sub> content</td></tr>
    <tr><td><strong>Saline/fluid dilution</strong> (e.g. flush line)</td><td>PCO<sub>2</sub> and HCO<sub>3</sub><sup>−</sup> both fall equally</td></tr>
    <tr><td><strong>Hypothermia</strong> (no temperature correction)</td><td>Spuriously higher PCO<sub>2</sub>, lower pH, higher PO<sub>2</sub></td></tr>
    <tr><td><strong>Hyperthermia</strong> (no temperature correction)</td><td>Opposite of hypothermia effects</td></tr>
    <tr><td><strong>Delayed analysis</strong></td><td>Continued cellular metabolism alters all values</td></tr>
  </tbody>
</table>
<p class="src">— Murray &amp; Nadel's, p. 2274–2278</p>

<!-- 8. CLINICAL APPLICATIONS -->
<h2>8. Clinical Applications</h2>

<h3>Emergency Medicine</h3>
<ul>
  <li><strong>Respiratory failure screening:</strong> A normal venous PCO<sub>2</sub> excludes hypercapnic failure. If severe hypercapnia or hypoxia, confirm with ABG.</li>
  <li><strong>DKA monitoring:</strong> VBG correlates well with ABG for pH/HCO<sub>3</sub><sup>−</sup>, avoiding repeated arterial punctures.</li>
  <li><strong>Toxicology:</strong> VBG used alongside electrolytes in phenol exposure and toxic ingestions.</li>
  <li><strong>Paediatric assessment:</strong> VBG used with glucose, ammonia, and metabolic panel in metabolic emergencies.</li>
</ul>

<h3>Critical Care / ICU</h3>
<ul>
  <li><strong>Acid-base monitoring:</strong> Central VBGs have excellent agreement with ABGs for acid-base assessment, including in patients in shock.</li>
  <li><strong>Sepsis:</strong> ScvO<sub>2</sub> monitoring (no longer a Surviving Sepsis Campaign recommendation as of 2016).</li>
  <li><strong>Haemorrhagic shock:</strong> VBG alongside TEG and type-and-screen prioritised in severe haemorrhagic shock.</li>
  <li><strong>COPD exacerbation:</strong> VBG recommended as initial blood gas; follow with ABG if severe hypercapnia or hypoxia.</li>
</ul>

<h3>Surgery / Anaesthesia</h3>
<ul>
  <li>Continuous SvO<sub>2</sub> monitoring via PAC in high-risk surgical patients provides real-time assessment of global perfusion adequacy.</li>
</ul>
<p class="src">— Tintinalli's, p. 304–310 &nbsp;|&nbsp; Rosen's EM, p. 2550 &nbsp;|&nbsp; Fishman's Pulmonary Diseases, p. 2060–2061</p>

<!-- 9. DECISION TABLE -->
<h2>9. VBG vs. ABG — When to Choose Which</h2>
<table>
  <thead><tr><th>Clinical Question</th><th>Recommended Test</th></tr></thead>
  <tbody>
    <tr><td>Is oxygenation adequate?</td><td>ABG or SpO<sub>2</sub> — VBG cannot assess this</td></tr>
    <tr><td>Is the patient hypercapnic?</td><td>VBG first; normal excludes hypercapnia. Confirm ABG if elevated or severe</td></tr>
    <tr><td>What is the acid-base status?</td><td>VBG is acceptable (pH ±0.03–0.05)</td></tr>
    <tr><td>Is bicarbonate normal?</td><td>VBG (HCO<sub>3</sub><sup>−</sup> ~2–3 mEq/L higher than arterial)</td></tr>
    <tr><td>Is lactate elevated?</td><td>VBG for screening; mildly elevated should be confirmed with ABG</td></tr>
    <tr><td>Is global O<sub>2</sub> delivery adequate?</td><td>Mixed SvO<sub>2</sub> from PAC (or ScvO<sub>2</sub> as a trend)</td></tr>
    <tr><td>Hypotensive / severely hypercapnic patient</td><td>ABG preferred; VBG unreliable in this setting</td></tr>
  </tbody>
</table>

<!-- 10. FORMULAS -->
<h2>10. Key Formulas</h2>

<h3>Winter's Equation — Expected PaCO<sub>2</sub> in metabolic acidosis</h3>
<div class="formula">PaCO<sub>2</sub> = (1.5 &times; [HCO<sub>3</sub><sup>−</sup>] + 8) &plusmn; 2</div>
<p>A measured PaCO<sub>2</sub> <em>above</em> the expected value indicates an inadequate ventilatory response to the metabolic acidosis.</p>

<h3>Fick Equation — Cardiac output from mixed venous saturation</h3>
<div class="formula">CO = VO<sub>2</sub> / (CaO<sub>2</sub> &minus; CvO<sub>2</sub>)</div>
<p>Requires SvO<sub>2</sub> measured at the pulmonary artery.</p>

<h3>Dead Space Fraction — Requires ABG</h3>
<div class="formula">V<sub>D</sub>/V<sub>T</sub> = (PaCO<sub>2</sub> &minus; P<sub>E</sub>CO<sub>2</sub>) / PaCO<sub>2</sub></div>
<p>P<sub>E</sub>CO<sub>2</sub> is the mean expired partial pressure of CO<sub>2</sub>, measured by metabolic monitor or capnography in mechanically ventilated patients.</p>

<!-- 11. SUMMARY TABLE -->
<h2>11. Summary Comparison Table</h2>
<table>
  <thead><tr><th>Feature</th><th>VBG</th><th>ABG</th></tr></thead>
  <tbody>
    <tr><td>Invasiveness</td><td>Low (venipuncture)</td><td>Higher (arterial puncture)</td></tr>
    <tr><td>Pain</td><td>Minimal</td><td>Moderate</td></tr>
    <tr><td>pH accuracy</td><td>Good (±0.03–0.05)</td><td>Reference standard</td></tr>
    <tr><td>PCO<sub>2</sub></td><td>~3–8 mmHg higher than arterial</td><td>Reference</td></tr>
    <tr><td>PO<sub>2</sub> / Oxygenation</td><td>&#10060; Not useful</td><td>&#9989; Required for O<sub>2</sub> assessment</td></tr>
    <tr><td>HCO<sub>3</sub><sup>−</sup></td><td>~2–3 mEq/L higher</td><td>Reference</td></tr>
    <tr><td>Lactate</td><td>Acceptable (screen); confirm if mildly elevated</td><td>More accurate</td></tr>
    <tr><td>Use in shock / hypotension</td><td>&#9888; Unreliable</td><td>&#9989; Preferred</td></tr>
    <tr><td>Mixed venous O<sub>2</sub> (SvO<sub>2</sub>)</td><td>Via PAC only</td><td>N/A</td></tr>
  </tbody>
</table>

<!-- REFERENCES -->
<div class="refs">
  <h3>References</h3>
  <ul>
    <li>Tintinalli's Emergency Medicine: A Comprehensive Study Guide, pp. 304–310</li>
    <li>Murray &amp; Nadel's Textbook of Respiratory Medicine, pp. 2270–2282</li>
    <li>Goldman-Cecil Medicine International Edition, p. 754</li>
    <li>Fishman's Pulmonary Diseases and Disorders, p. 2060</li>
    <li>Miller's Anesthesia 10th Edition, pp. 833–835</li>
    <li>Rosen's Emergency Medicine: Concepts and Clinical Practice, pp. 2550–2551</li>
    <li>Harrison's Principles of Internal Medicine 22E (2025)</li>
  </ul>
</div>

<div class="footer">Venous Blood Gases — Orris Medical Reference &nbsp;|&nbsp; June 2026</div>

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