Ventilatory drive

Some patients lose the ability to increase the rate or depth or respiration in response to persistent hypoxemia. This decreased ventilatory drive may be due to abnormal peripheral or central respiratory receptor responses. This relative hypoventilation leads to hypercapnia in this situation the central respiratory response to a chronically increased PaC02 can be blunted. Because these changes in Pa02 and PaC02 are subtle and progress over many years, the pH is usually near normal because the kidneys compensate by retaining bicarbonate. 

At Paco2 of 10 kPa This line is plotted above and to the right of the first and demonstrates the effect of a coexisting hypercarbia on hypoxic ventilatory drive. 

Sobotka PA, Liss HP, Vinik AI. Impaired hypoxic ventilatory drive in diabetic patients with autonomic neuropathy. J Clin Endocrinol Metab 1986 62 658-663. 

Regensteiner JG, Woodard WD, Hagerman DD, Weil JV, Pickett CK, Bender PR, Moore LG (1989) Combined effects of female hormones and metabolic rate on ventilatory drives in women. J Appl Physiol 66 808-813... 

Although diazepam does not have anticholinergic properties, it is possible to reverse diazepam-induced delirium by the use of cholinesterase inhibitors, such as physostig-mine however, physostigmine can on occasion induce severe arterial hypertension, especially if the dose exceeds 2 mg intravenously. In healthy volunteers sedated with diazepam, an increase in awareness was established with the use of physostigmine, but there was also a reduction in ventilatory drive (SEDA-10, 119). 

Opioids are potent respiratory depressants, causing a dose-dependent decrease in respiratory frequency, tidal volume and minute ventilation and increased arterial partial pressure of carbon dioxide (PaC02) (Carvey 1998). Opioids depress chemosensors in the brainstem, decreasing the ventilatory response to carbon dioxide. Opioids also depress rhythmicity in the dorsal respiratory group in the nucleus tractus solitarius, attenuating the respiratory cycle. Opioids, however, do not diminish hypoxic ventilatory drive. Significant elevations in Paco2 can result in increased ICP after opioid administration. 

Dopamine often is recommended as the initial catecholamine in sepsis because it increases blood pressure by increasing myocardial contractility and vasoconstriction. Dopamine has been described to have dose-related receptor activity at DAj-, DA2-, fi -, and i-receptors. Unfortunately, this dose-response relationship has not been confirmed in critically ill patients. In patients with septic shock, there is a great overlap of hemodynamic effects even at doses as low as 3 mcg/kg per minute. Tachydysrhythmias are common owing to the release of endogenous norepinephrine by dopamine entering the sympathetic nerve terminal. Dopamine may increase the PAOP through pulmonary vasoconstriction. This drug also may depress ventilation and worsen hypoxemia in patients dependent on the hypoxic ventilatory drive. 

The stroke physician will also monitor the patient s breathing pattern for signs of airway obstruction or impaired ventilatory drive, air escaping from the side of the facial paresis, unequal palpebral fissure (as occurs with ptosis on the side of a Homer s), or impaired lid closure on the side of facial weakness. 

In elderly patients, especially those with multisystemic diseases, hypothyroidism may become severe and fife threatening. Myxedema coma represents the most extreme form of severe hypothyroidism. This medical emergency may occur when severe hypothyroidism is complicated by trauma, infection, myocardial infarction, cold exposure, or administration of hypnotics or opiates, medications that suppress central nervous system function, particularly ventilatory drive. It typically presents in older women in winter. The main clinical features are hypothermia and a variable degree of altered consciousness (Iglesias et ai, 1999). Serum T4, TSH and cortisol should be measured to confirm the diagnosis and evaluate adrenal reserve. When there is a reasonably high level of suspicion, treatment should not be delayed to await laboratory results. 

In 1992, Sloan et al. conducted breath-hold experiments with 20 healthy volunteers. The ingestion of a lozenge containing 11 mg of menthol signi cantly increased the hold time, indicating a depression of the ventilatory drive. It was later postulated by Eccles (2000) that in addition to chemoreceptors detecting oxygen and carbon dioxide in the blood, cold receptors in the respiratory tract may also modulate the drive to breathe. 

Ketamine is a rapid-acting dissociative anesthetic first approved 40 years ago for animals and humans, and in continuous use since. At anesthetic doses (several mg/kg) it produces a cataleptic state with nystagmus and intact corneal and light reflexes, and maintenance of ventilatory drive as well as blood pressure even in hypovolemic subjects [1]. The latter properties make it a potentially attractive alternative to morphine or other opioids in civilian emergency, mass... 

Swanson GD, Whipp BJ, Kaufman RD, Aqleh KA, Winter B, Bellville JW. Effect of hypercapnia on hypoxic ventilatory drive in carotid hody-resected man. J Appl Physiol 1978 45 971-977. 

Kryger M, McCullough RE, Doekel R, Collins DD, Weil jy Grover RF. Excessive polycythemia of high altitude Role of ventilatory drive and lung disease. Am Rev... 

In mammals the carotid bodies are situated at the bifurcations of the cormnon carotid arteries into their internal and external branches and are irmervated by the sinus nerve, a branch of cranial nerve EX (Fig. 1). The aortic bodies in mammals, on the other hand, are located in the region of the aortic arch and the roots of the major arteries of the thorax. Their afferent fibers nm in the aortic nerve, a branch of the vagus (Fig. 1). They appear to make little contribution to the resting ventilatory drive in eucapnic normoxia and may not contribute to the hypoxic ventilatory response in many species. It would appear that they participate almost exclusively in cardiovascular reflexes in this group (see Ref. 25 for review). Glomus tissue that may... 

The function of the ventilatory pump is critically dependent on three factors the respiratory workload, the respiratory muscle strength, and the ventilatory drive (Fig. 1). Chronic hypercapnic respiratory failure can result from one or more of these abnormalities inadequate ventilatory drive, excessive respiratory load, and inadequate inspiratory muscle... 

Other metabolic factors contributing to PMV include hypophosphatemia and hypomagnesemia, both of which have been associated with diminished diaphragmatic function. Hypothyroidism is an uncommon cause of ventilator dependency (27), being associated with respiratory muscle weakness as well as altered ventilatory drive and upper airway obstruction. Hypothyroidism is a potentially treatable cause of failure to wean and it should be considered in patients with prolonged ventilator dependence. 

Only a small proportion of patients fail to wean from mechanical ventilation, but they require a disproportionate amount of resources. Weaning failure has been extensively studied in the clinical literature and several factors are likely to contribute to it. These factors include inadequate ventilatory drive, respiratory muscle weakness, respiratory muscle fatigue, increased work of breathing, or cardiac failure. There is accumulating... 

Schucher B, Laier-Groeneveld G, Huettemann U, et al. Effects of intermittent self-ventilation on ventilatory drive and respiratory pump fimction. Med Klin (Munich) 1995 90(1 suppl 1) 13-16. 

The most common causes of failure to wean include chronic obstructive pulmonary disease (COPD) exacerbations, neuromuscular diseases, h) oxic respiratory failure, post surgical complications (2), and heart failure. Weaning from the tracheostomy must consider the balance of respiratory muscle function and work of breathing. The work of breathing is determined by ventilatory demand, compliance of the lungs and chest wall, airway resistance, and intrinsic positive end-expiratory pressure (PEEPi). Adequacy of ventilatory drive and neuromechanical output can be assessed from the respiratory rate, airway occlusion pressure at 100 milliseconds (Po.i), maximum inspiratory pressure (MIP), and maximum voluntary ventilation (MW). 

Table 1 Adverse Effects of Malnutrition on Respiratory Functions Altered ventilatory drive...

Obesity reduces the emergency relief value and, to a lesser extent, the functional residual capacity (FRC). In more severe disease, it also reduces the vital capacity and the total lung capacity. Obesity is also associated with an increase in respiratory resistance and a reduction in thoracic cage compliance. It doubles the work of breathing and quadruples the energy cost of breathing (11). To meet these demands, ventilatory drive is doubled with a higher respiratory rate and a smaller tidal volume. When the obese patient is awake and supine, the compliance of... 

A previous study (14) has suggested that nocturnal MTV could be used as an interim measure in subjects with severe OSA and hypercapnia until ventilatory decompensation is reversed (possibly by alterations in ventilatory drive and ventilatory responses to hypercapnia and hypoxia) and CPAP therapy can then be used long term. Others (30) have shown that a proportion of patients may be switched over to CPAP once respiratory failure has been controlled. CPAP therapy from the start, rather than bi-level ventilation followed by CPAP, may be just as effective (particularly improving sleep architecture and arousals) and potentially more cost-effective in patients with OHS, even if blood gases are not corrected immediately. 

Malnutrition, acidosis, electrolyte disturbances, cachexia, infection, fatigue, and muscle dysfunction, aU exacerbate ventilatory insufficiency. Narcotics, sedatives, and supplemental oxygen reduce ventilatory drive and exacerbate alveolar hypoventilaticm. 

Closed systems NIPPV are unnecessary unless ventilatory drive is blunted by oxygen therapy, sedative medications, or excessive hypercapnia, which can lead to excessive air leakage from the nose or mouth (4). If necessary, one can provide an essentially closed system of ventilatory support by using a lipseal (Respironics International Inc., Murrysville, Pennsylvania, U.S.), placing cotton pledgets in the nostrils, and sealing the nostrils with a... 

Figure 1 A beagle dog was anesthetized with propofol and etomidate and intubated. One 250-mL breath of ammonia vapor above an 8-M ammonia solution was administered to the dog. The end-tidal COj shows an apnea followed by rapid, shallow breathing. The arterial pressure tracing demonstrates the short-lived decrease in heart rate and attendant hypotension. The transpulmonary pressure illustrates that the apnea and tachypnea was due to the absence of ventilatory drive, rather than airway occlusion.

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