Take Miller's anesthesia as reference and give effects of opoids on respiratory system in sentences
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I now have all the content needed. Let me compile the comprehensive answer from Miller's Anesthesia 10e.
Effects of Opioids on the Respiratory System
(Reference: Miller's Anesthesia, 2-Volume Set, 10e - Chapter 22)
1. Overview
Opioids can affect respiration through multiple mechanisms, and opioid overdose can lead to fatal hypoxia caused by prolonged apnea. Opioid-induced respiratory depression is caused by its suppressive effect on the pontomedullary respiratory center, and is characterized by slow, shallow, and irregular breathing, which in severe cases leads to respiratory arrest. Opioids also impair chemoreflexes and upper airway patency and induce muscle rigidity, sedation, and sleep-disordered breathing, all of which compromise patient outcomes.
2. Effects on Airways (Antitussive Actions)
The antitussive actions of opioids are well known and are central in origin. Opioids blunt or eliminate somatic and autonomic responses to tracheal intubation and allow patients to tolerate endotracheal tube placement without coughing or bucking. Remifentanil (at an effect-site concentration of 2 ng/mL) can suppress coughing induced by extubation after propofol or sevoflurane anesthesia.
Morphine has a depressant effect on respiratory mucus secretion in human bronchi, which is one of the most important defenses against respiratory tract infections; however, morphine had no effect on the beating frequency of nasal cilia in vitro. A sedative dose of morphine (0.1 mg/kg) was associated with increased incidence of pharyngeal dysfunction and discoordinated breathing and swallowing, a combination that impairs airway protection and potentially increases the risk for pulmonary aspiration.
3. Opioid-Induced Cough (Paradox)
Despite their antitussive properties, opioids can paradoxically induce cough. Equipotent boluses of sufentanil 0.3 mcg/kg and fentanyl 3 mcg/kg both increase the incidence of cough, although the incidence and severity of coughing with sufentanil are less than with fentanyl. The incidence of fentanyl-induced cough (100-150 mcg given IV) decreased from 18% to 1.3% when the injection time was increased from 2 seconds to 30 seconds. Lidocaine 1.5 mg/kg administered 1 minute before fentanyl effectively suppresses opioid-induced cough, and a meta-analysis identified 0.5 mg/kg as the lowest effective lidocaine dose. Propofol, alpha-2 agonists (clonidine, dexmedetomidine), beta-2 agonists (terbutaline, salbutamol), and NMDA-receptor antagonists (ketamine, dextromethorphan) are also effective in suppressing fentanyl-induced cough.
In summary, opioids engender potent antitussive properties that help blunt airway responsiveness to endotracheal intubation, but depending on the class of opioid and rate of administration, they may evoke a brief cough that can be subverted by preadministered agents such as lidocaine.
4. Respiratory Depression - The Primary Adverse Effect
The respiratory-depressant actions of opioids represent their most serious adverse effect. Opioids activating the mu-receptor cause dose-dependent depression of respiration, primarily through a direct action on brainstem respiratory centers. Although the preBötzinger complex (PreBötC), the main region of respiratory rhythm-pattern generation, has been known to be the main target of opioid-induced respiratory depression, it was demonstrated that the PreBötC partially mediates opioid effects on respiratory phase timing but does not mediate the opioid-induced depression of respiratory rate.
Activation of the mu-opioid receptor in the caudal medullary raphe region, which is important for regulating pain and respiration, inhibits the ventilatory response to hypercapnia. Polymorphism of the mu-opioid receptor at nucleotide position 118 does not significantly change the susceptibility to the respiratory depressive effect of M6G, which suggests that analgesia and respiratory depression may be mediated by different signal transduction mechanisms activated by the mu-opioid receptor.
5. Effects on CO2 Response and Chemoreflexes
The stimulatory effect of CO2 on ventilation is significantly reduced by opioids, and the pressure of end-tidal carbon dioxide (PetCO2) is increased. Opioids also decrease hypoxic ventilatory drive and induce characteristic changes in respiratory pattern. Plasma fentanyl concentrations of 1.5-3.0 ng/mL are associated with significant decreases in CO2 responsiveness, and when moderately large doses (20-50 mcg/kg or greater) of fentanyl are used, the potential need for postoperative mechanical ventilation should be anticipated. In healthy humans, the EC50 for depression of minute ventilation with remifentanil and alfentanil was 1.17 ng/mL and 49.4 ng/mL, respectively. Fentanyl 1 mcg/kg and remifentanil 0.5 mcg/kg had similar maximum decreases in minute ventilation (~50%), but onset and recovery from ventilatory depression were faster with remifentanil.
6. Effects on Respiratory Rate and Pattern
Respiratory rate is usually drastically decreased in opioid overdose, although hypoxic CNS insult can counter this effect. The prolonged expiratory time in the respiratory cycle induced by opioids frequently results in greater reductions in respiratory rate than in tidal volume. High doses of opioids usually eliminate spontaneous respirations without necessarily producing unconsciousness, and patients receiving high doses of opioids may still be responsive to verbal command and often breathe when directed to do so.
7. Postoperative Respiratory Function
Inadequate postoperative pain relief in the thoracoabdominal region can inhibit deep breathing and cause shallow respiration, leading to postoperative respiratory dysfunction including atelectasis; therefore, opioids can prevent or correct respiratory impairment as a basic component of postoperative analgesia. However, opioids can also dose-dependently depress respiration - the incidence of postoperative opioid-induced respiratory depression was reported to be 0.1%-37%, depending on the route of opioid administration, the type of opioid, the definition and method of monitoring, and the prospective versus retrospective nature of the study.
8. Mechanism of Respiratory Depression
By use of knockout mice and pharmacologic approaches, G-protein-gated inwardly rectifying K+ channels were shown to contribute to respiratory depression by mu-opioid receptors. A major component of the excitatory synaptic drive necessary for respiratory rhythmogenesis is mediated by AMPA-type glutamate receptors, leading to investigations of ampakine therapy to alleviate opioid-induced respiratory depression. 5-HT released from the raphe nuclei potently alters the excitability of respiratory motoneurons in the PreBötC and other brainstem respiratory nuclei, and activation of 5-HT1A receptors with befiradol alleviated fentanyl-induced respiratory depression in rats.
9. Factors Affecting Opioid-Induced Respiratory Depression
The following factors increase the magnitude and/or duration of opioid-induced respiratory depression (Box 22.2 in Miller's):
- High dose of opioid
- Sleep state
- Neonates (morphine easily penetrates the incomplete blood-brain barrier in neonates and infants, producing greater respiratory depression on a weight basis than in adults)
- Old age (older patients are more sensitive to the respiratory-depressant effect and experience higher plasma concentrations on a weight basis)
- CNS depressants (inhaled anesthetics, alcohol, barbiturates, benzodiazepines, most IV sedatives and hypnotics) - though droperidol, scopolamine, and clonidine do NOT enhance the respiratory-depressant effects of opioids
- Hyperventilation/hypocapnia (enhances and prolongs postoperative respiratory depression by increasing brain opioid penetration and decreasing removal via reduced cerebral blood flow)
- Respiratory acidosis
- Hyperoxia (respiratory depression by remifentanil was more pronounced while inhaling 50% O2 than in normoxia; additionally, pulse oximetry may remain normal during the first minutes of respiratory depression in hyperoxia, masking the condition)
- Decreased clearance (reduction of hepatic blood flow, renal insufficiency - as active metabolites of hydromorphone and the morphine metabolite M6G accumulate)
Opioid-induced respiratory depression can also be affected by the patient's original respiratory status - in patients who hyperventilate because of anxiety or pain, even small doses of IV opioids can result in transient apnea because of acute shifts in apneic thresholds.
10. Renarcotization Risk
All opioid agonists with a longer plasma half-life than naloxone have a hypothetical potential for renarcotization with time, especially when a bolus dose of naloxone is used to treat opioid-induced respiratory depression. Overall, opioids exert their primary respiratory depressant effects through mu-receptor activation in the brainstem respiratory center - dose-dependent opioid respiratory depression increases the apneic threshold and reduces the respiratory stimulatory drive of both CO2 and hypoxia.
Source: Miller's Anesthesia, 2-Volume Set, 10e, Chapter 22 - Respiratory Effects of Opioids (pp. 2751-2764)
Tell the respiratory depression doses of opiods
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I now have all the key dose data from Miller's. Here is the consolidated answer:
Respiratory Depression Doses of Opioids
(Reference: Miller's Anesthesia, 2-Volume Set, 10e - Chapter 22)
Fentanyl
- Plasma fentanyl concentrations of 1.5 to 3.0 ng/mL are associated with significant decreases in CO2 responsiveness.
- When moderately large doses of 20 to 50 mcg/kg or greater of fentanyl are used, the potential need for postoperative mechanical ventilation should be anticipated.
- In healthy volunteers, fentanyl 1 mcg/kg produced a maximum decrease in minute ventilation of approximately ~50% (similar in magnitude to remifentanil 0.5 mcg/kg), but onset and recovery were slower.
- Fentanyl >2.9 mcg/kg produces dose-dependent respiratory depression that results in apnea (in contrast to buprenorphine, which reaches a ceiling effect).
- In PCA, the optimal demand dose for opioid-naive patients is 40 mcg for fentanyl - doses above this carry a higher risk of respiratory depression.
Remifentanil
- The EC50 (concentration producing 50% depression of minute ventilation) for remifentanil is 1.17 ng/mL.
- Remifentanil 0.5 mcg/kg (IV bolus) produces a ~50% maximum decrease in minute ventilation - comparable to fentanyl 1 mcg/kg, but with a faster onset and faster recovery.
- Respiratory depression by remifentanil 50 mcg infused over 60 seconds is more pronounced under hyperoxia (50% O2) than normoxia, as measured by minute ventilation, PetCO2, and respiratory rate.
- When combined with propofol, even low concentrations of both drugs produce a strikingly synergistic and potentially severe respiratory depression. Combined with midazolam, respiratory depression is similarly enhanced.
Alfentanil
- The EC50 for depression of minute ventilation with alfentanil is 49.4 ng/mL.
Morphine
- A bolus dose of 100 mcg/kg followed by a continuous infusion of 30 mcg/kg/h suppresses resting inspired minute ventilation and increases PetCO2 (as shown in Fig. 22.19 of Miller's).
- In PCA, the optimal demand dose for opioid-naive patients is 1 mg morphine - exceeding this increases the risk of respiratory depression.
- Morphine can produce greater respiratory depression in neonates than adults on a weight basis, because it easily penetrates the immature blood-brain barrier.
- Oxycodone 20 mg orally in opioid-naive healthy volunteers induced significant PetCO2 increase, further potentiated by ethanol co-administration.
Buprenorphine
- Buprenorphine produces depression of minute ventilation that levels off at doses higher than 3.0 mcg/kg to approximately 50% of baseline (ceiling effect on respiratory depression).
- This contrasts sharply with full agonists like fentanyl, where respiratory depression continues in a dose-dependent manner progressing to apnea at doses >2.9 mcg/kg.
- Despite this theoretical ceiling effect, a systematic review reported that in clinical settings, buprenorphine can still cause significant adverse effects on respiratory function.
Sufentanil
- Equipotent boluses of sufentanil 0.3 mcg/kg increase the incidence of cough-related respiratory effects, though the incidence and severity are less than with fentanyl 3 mcg/kg.
Tramadol
- Analgesic doses of tramadol produce less respiratory depression than other opioids, partly because of its nonopioid receptor-mediated mechanism of action. Tramadol is one-fifth to one-tenth as potent as morphine.
Summary Table
| Opioid | Key Respiratory Depression Threshold |
|---|---|
| Fentanyl | 1.5-3.0 ng/mL plasma = decreased CO2 response; >2.9 mcg/kg = apnea; 20-50 mcg/kg = mechanical ventilation anticipated |
| Remifentanil | EC50 = 1.17 ng/mL; 0.5 mcg/kg = ~50% decrease in minute ventilation |
| Alfentanil | EC50 = 49.4 ng/mL |
| Morphine | 100 mcg/kg bolus = suppressed minute ventilation + raised PetCO2 |
| Buprenorphine | Ceiling at >3.0 mcg/kg (~50% of baseline) - no apnea even at high doses |
Source: Miller's Anesthesia, 10e, Chapter 22 (pp. 2755-2875)
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